Vibracoring off the R/VG.K. Gilbert

A submarine sediment sample is cored by a vibrating barrel in depths of water up to 30 meters. [larger version]

The USGS Marine Operations Group in St. Petersburg designed and constructed a pneumatic vibracoring rig in 1995 to take underwater sediment cores up to 20 feet long inside 3 inch aluminum barrels. In 2002 the rig was modified to use an electric motor-driven, eccentric-cam vibrator (Rossfelder model P3) to replace the pneumatic vibrator, and the boat was rewired to 3-phase electrical service to operate it. This is a better corer both in terms of sediment penetration and in terms of ease of use.

The vibracoring rig consists of two guyed, 22 ft-tall, cross-braced masts that support the vibrating head as it slides down two guide wires to drive the aluminum core barrel through an opening in the center of the rig's 6 ft X 5 ft, 400-pound steel base-frame into the bottom sediments. The rig is assembled on the deck of the R/VG.K. Gilbert at the start of a project, and deployed over the side for underwater coring in waters between 1 and 30 meters deep using the boat's Hiab SeaCrane. Occasionally the sediments will resist full 7-meter penetration (called "depth of refusal") and it may be necessary to use a water pump with an adapter attached to the top of the barrel to water-jet the barrel down to the refusal depth and re-start coring from there to obtain a full, 20 ft-long sediment core.

It takes four crew to handle the rig on deck of the R/VG.K. Gilbert. After the vessel has been securely two-point anchored, the entire 800-pound rig is lifted over the side with the crane and lowered to the bottom where the vibrator is turned on to drive the core barrel into the sediments. Progress is continuously monitored using a penetrometer which sends a signal to an onboard computer that graphically depicts how far and how fast the core barrel is descending. When full penetration (or depth of refusal) is obtained, the vibracore motor is shut off and the crane is used to pull the core barrel out of the mud. The rig is pulled off the bottom and brought back aboard. The mud stays in the barrel because there is a one-way poppet valve at the top of the barrel to create suction and a brass core-catcher is riveted to the bottom of the barrel. The core-catcher acts like a set of fingers that close if the mud tries to slide out.

Vibracore rigs have been used aboard the R/VG.K. Gilbert to take thousands of sediment cores along the southeastern coast of the United States (from South Carolina to Texas), and aboard other vessels off the coasts of New Jersey and Belize, as well as in the inland lakes of Idaho and Utah. The cores are returned to the USGS St. Petersburg Coastal and Marine Science Center Sediment Lab where they are split lengthwise and analyzed for stratigraphic horizons and sediment types to ground-truth seismic data. Alternatively, sediment cores are discretely sampled at various depths for chemical and biological analyses. These data indicate the location of sand resources for barrier island and beach renourishment projects, and provide a historical record of deposition, storm records, and anthropogenic impacts.

The vibracore rig is deployed from the R/VG.K. Gilbert by USGS scientists. [larger version]